The present invention relates to a water-dispersible binder for cationic electrocoating finishes, obtained by reaction of
(A) epoxy resins containing low molecular weight aromatic groups and having an epoxide equivalent weight of less than 375 with PA0 (B) aliphatic and/or alicyclic polyfunctional alcohols or carboxylic acids having a number average molecular weight of less than 350, further reaction of the product of (A) and (B) with PA0 (C) a polyfunctional alcohol having a molecular weight of more than 350, addition at the epoxide groups taking place so that the reaction products of (A) and (B) contain an amount of aromatic groups of from 10 to 45% by weight, calculated as phenylene groups, and further modification of the resulting intermediate with PA0 (D) primary and/or secondary amines or ammonium salts in order to achieve the required water-dispersibility. PA0 (A) epoxy resins containing low molecular weight aromatic groups and having an epoxide equivalent weight of less than 375 with PA0 (B) aliphatic and/or alicyclic polyfunctional alcohols or carboxylic acids having a number average molecular weight of less than 350, further reaction of the product of (A) and (B) with PA0 (C) a polyfunctional alcohol having a molecular weight of more than 350, addition at the epoxide groups taking place so that the reaction products of (A) and (B) contain an amount of aromatic groups of from 10 to 45% by weight, calculated as phenylene groups, and further modification of the resulting intermediate with PA0 (D) primary and/or secondary amines or ammonium salts in order to achieve the required water-dispersibility, wherein, as component (C), 5 to 40% by weight, based on the sum of components (A), (B), (C) and (D), of a linear polyalkylene ether diol are employed which has on average one terminal primary and one terminal secondary hydroxyl group per molecule and corresponds to the general formula ##STR6## in which n denotes 1 to 4, preferably 1, and m denotes 4 to 35, preferably 5 to 20. PA0 (A) epoxy resins containing low molecular weight aromatic groups and having an epoxide equivalent weight of less than 375 with PA0 (B) aliphatic and/or alicyclic polyfunctional alcohols or carboxylic acids having a number average molecular weight of less than 350, further reaction of the product of (A) and (B) with PA0 (C) a polyfunctional alcohol having a molecular weight of more than 350, addition at the epoxide groups taking place so that the reaction products of (A) and (B) contain an amount of aromatic groups of from 10 to 45% by weight, calculated as phenylene groups, and further modification of the resulting intermediate with PA0 (D) primary and/or secondary amines or ammonium salts in order to achieve the required water-dispersibility, and the binders being rendered self-crosslinkable as a result of reaction, or the bath containing an additional crosslinking agent, wherein, as component (C), for the preparation of the binder, 5 to 40% by weight, based on the sum of components (A), (B), (C) and (D), of a linear polyalkylene ether diol are employed which has on average one terminal primary and one terminal secondary hydroxyl group per molecule and corresponds to the general formula ##STR7## in which n denotes 1 to 4, preferably 1, and m denotes 4 to 35, preferably 5 to 20.
It is known that cationic water-dispersible synthetic resins can be used as binders for electrocoating finishes. Thus, for example, German Offenlegungsschrift No. 2,701,002 describes a resin of this type, which is a reaction product of a polyepoxide having a molecular weight of at least 350, a secondary amine and an organic polyol containing at least two alcoholic primary hydroxyl groups. The resins are formed as a result of chain-lengthening of high molecular weight polyepoxides having at least two epoxide groups per molecule. Chain-lengthening is achieved by means of an organic polyol, and water-dispersibility is achieved by means of an addition reaction with a secondary amine.
This and other known synthetic resins for cathodic electrocoatings are frequently used for prime coats, ie. the articles coated with them receive an additional top coat. However, it is a disadvantage that only coatings having a relatively small layer thickness can be achieved with the resins known hitherto. Thus, German Offenlegungsschrift No. 2,701,002 states that layer thicknesses of only 11.4 to 18 .mu.m can be achieved. When a coating is subject to particularly high requirements in respect of corrosion-resistance and surface quality, as is the case, for example, in the coating of automobiles and other high-quality goods, it has therefore been usual hitherto to apply a so-called filler as an additional layer between the electrocoating prime coat and the top coat. This is complicated and expensive. Hence, it is desirable to improve the electrocoating process so that greater layer thicknesses can be achieved in this process. In the attempt to increase the layer thickness, problems have resulted from the fact that, when the deposition voltage is increased above the breakdown voltage, surface irregularities occur in the film as a result of rupture of the layer. Increasing the coating time likewise results in an increase in the layer thickness; however, this increase cannot be continued as desired, since, because of the electrical resistance of the deposited film, there is normally an upper limit to the layer thickness, at which virtually no further increase in the layer thickness is achieved no matter how long coating is carried out.
German Offenlegungsschrift No. 3,108,073 corresponding to U.S. patent application Ser. No. 432,960, filed Sept. 27, 1982 discloses a water-dispersible binder for cationic electrocoating finishes which, when used, gives a deposited film which is relatively thick. Although this known binder gives coatings having excellent properties, it is nevertheless desirable to improve the flexibility of the resulting coatings by modifying the binder.
Surprisingly, it has now been found that coatings having improved properties in respect of flexibility are obtained if the binder used contains a proportion of a linear polyether diol which contains a secondary hydroxyl group in addition to a primary hydroxyl group.
The use of a diol containing primary and secondary hydroxyl groups in combination was not obvious since, on the basis of German Offenlegungsschrift No. 2,701,002, the skilled worker had to assume that exclusively diols with primary hydroxyl groups were suitable for the use in question. On page 10 of Offenlegungsschrift No. 2,701,002, it is stated that it is important for the alcoholic hydroxyl groups to be primary and not secondary or tertiary. Furthermore, the danger of gelling of the resin when diols having secondary hydroxyl groups are used is pointed out.